EFFECT OF DISLOCATION ACTIVITIES ON CRACK EXTENSION IN IONIC-CRYSTALS

The effect of dislocation activities on crack extension has been inves tigated using NaCl, KCl and Na-Cl-Br mixed crystals. In NaCl and KCl c rystals, {110}(110) slip systems operate actively around a crack tip a t room temperature, while the threshold stress intensity K-IC for crac k extension is very small, less than 0.1 MPa m(1/2). When test tempera tures are lowered to 77 K-IC the dislocation activities are reduced an d the K-IC values are increased. The K-IC values are also increased at room temperature when the crystals are hardened by the partial substi tution of Br for Cl in order to reduce dislocation activities. When te mperatures are raised above 400 K, {001}(110) slip operation in additi on to the abundant operation of {110}(110) slips takes place around a crack tip and the K-IC values are markedly increased. Such peculiar ef fects of dislocation activities on K-IC values are discussed in connec tion with the activities of shielding and antishielding dislocations e stimated from computer simulations as well as the experimental observa tions.